1. Field of the Invention
The present invention relates to a process for preparing a thermostable catalyst carrier based on TiO2/SiO2 mixed oxides comprised of 0.5 to 95 wt % SiO2 and the balance as TiO2, each referring to the completely calcined product.
2. Description of the Prior Art
Supported catalysts have economically been very important for a long time and make up the largest tonnage among the heterogeneous catalysts. They are used among others for working up fossil raw materials, upgrading intermediates, and making new chemical products. Owing to the worldwide increasing concern for the environment and the legal requirements in connection therewith, these catalysts have also rapidly gained importance for the emission control of exhaust gas.
For example, in industrial countries power stations and automobiles utilising fossil fuels are only approved if they are provided with appropriate supported catalysts for controlling emissions. Said supported catalysts are employed for denitrating (DeNOx) and desulphering flue gases and cleaning the exhaust gases of automobiles.
Conventional catalyst carriers for these applications are porous powders or moulded articles based on Al2O3, SiO2, MgO, TiO2, and ZrO2. In modern processes, e.g. for cleaning exhaust gases, metallic or ceramic honeycombs coated with oxidic carrier materials are increasingly employed. The catalyst carrier or honeycomb coating has the function among others to enlarge the surface of the active component because the catalyst activity usually increases as the catalyst surface increases.
Naturally, the metal oxides used as carriers, such as Al2O3, SiO2, MgO, TiO2, and ZrO2 have different properties, e.g. with regard to possible surfaces, pore volumes, acidity, and chemical inertia. Among these metal oxides TiO2 is known for being chemically inert and, consequently, for being especially suitable as a catalytic converter utilised for detoxifying aggressive gases containing e.g. SOx or NOx at high temperatures.
Conventional TiO2 materials available on the market have the disadvantage that they have small surfaces and pore volumes so that their use is limited. These commercially available catalyst carriers based on titanium dioxide are mostly produced by precipitation reactions of titanium-containing salts or by flame pyrolysis. When using the latter method with TiCl4, titanium dioxide is obtained, which consists of a crystalline mixed phase (anatase/rutile, approx. 70%/30%) presenting a surface of approx. 58 m2/g (commercial product e.g. Degussa P 25). When using hydrolysis and calcination with titanium tetra(isopropylate), the resultant titanium dioxide, too, is comprised of a crystalline mixed phase having a surface of 106 m2/g.
Among the three natural varieties of TiO2 (anatase, brookite, rutile) rutile exhibits the highest thermodynamic stability. Rutile is not in demand as a catalyst but is rather employed in large quantities as a white pigment. For catalytic uses anatase is the appropriate variety. Anatase readily converts into rutile at temperatures above 600° C., and thus is of no use for catalysis.
Today pure-phase catalyst carriers based on TiO2 are preferred for heterogeneous catalysis. Said carriers are to have large surfaces and as few disturbing anions (e.g. SO42− or Cl−) as possible, i.e. less than 250 ppm.
The alkali content, especially sodium and potassium, and the alkaline-earth content, especially calcium and magnesium, should be less than 50 ppm each, whereas the iron content should be less than 100 ppm so that the effectiveness of the catalytically active phase when subject to heat is not destroyed by the labile chemical lattice defects.